1. Field of the Invention
The invention relates to a method of sampling an electrical lamp parameter of a high pressure gas discharge lamp operating at high frequencies. The invention also relates to a circuit for sensing the electrical lamp parameter according to this method.
2. Description of the Prior Art
High pressure discharge (HID) lamps, such as mercury vapor, metal halide and high pressure sodium lamps, are typically operated with a magnetic ballast at or slightly above normal power line frequencies, e.g. 60-100 Hz. It would be desirable to provide an electronic ballast which operates HID lamps at high frequencies at above about 20 kHz. High frequency ballasts are becoming increasingly popular for low pressure mercury vapor fluorescent lamps. The high frequency operation permits the magnetic elements of the ballast to be reduced greatly in size and weight as compared to a conventional low frequency magnetic ballast.
A major obstacle to the use of high frequency electronic ballasts for HID lamps, however, is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at the minimum, cause flicker of the arc which is very annoying to humans. In the worst case, acoustic resonance can cause the discharge arc to extinguish, or even worse, stay permanently deflected against and damage the wall of the discharge vessel, which will cause the discharge vessel to rupture.
The article "An Autotracking System for Stable Hf Operation of HID Lamps", F. Bernitz, Symp. Light Sources, Karlsruhe 1986, discloses a controller which continuously varies the lamp operating frequency about a center frequency over a sweep range. The sweep frequency is the frequency at which the operating frequency is repeated through the sweep range. The controller senses lamp voltage to evaluate arc instabilities. A control signal is derived from the sensed lamp voltage to vary the sweep frequency between 100 Hz and some Khz to achieve stable operation. However, this system has never been commercialized.
U.S. Pat. No. 5,569,984 (Holstlag) discloses a method of avoiding arc instabilities by evaluating deviations in an electrical parameter of the lamp. In Holstlag, frequency sweeps are used to detect a stable operating frequency, but the lamp is then operated at a fixed frequency as long as the discharge arc remains stable at that frequency. This is in contrast to the method of the above-referenced Bernitz article, which continuously sweeps the lamp operating frequency during operation.
Both techniques have in common that an electrical parameter of the lamp is sensed. Holstlag '984 teaches that lamp voltage can be used, but that this has the disadvantage that the sampling moment must be triggered at a definite point within the lamp voltage waveform. Holstlag teaches that sensing the conductivity is favorable, as having a much higher signal-to-noise ratio than either the lamp current or voltage alone. Holstlag further teaches that using the lamp conductivity is favorable, at least from the standpoint of not requiring triggering at a definite point in the period of the lamp voltage. When using conductivity, the lamp voltage and current need to be taken simultaneously, in order for the noise in the signal to cancel, but the simultaneous sample need not be keyed to a particular point in the lamp voltage period.